Justice John Paul Stevens said Friday that he would retire from the Supreme Court this summer, setting up an election-year confirmation battle that could stir an already volatile political atmosphere.

The resignation isn't expected to change the ideological balance on the Supreme Court, given Justice Stevens's position atop the liberal bloc. But it does give President Barack Obama a second chance to shape the high court, most likely a liberal who could be in a position to serve on the court for decades to come.

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Tom Goldstein, a Washington lawyer and co-founder of a blog about the Supreme Court, called Ms. Kagan the "prohibitive front-runner."